Null check field names in QueryStringQueryBuilder

[dbwiddis]

Description

Adds a check for a null value before getting text on a field name, which would otherwise throw an IllegalStateException. This results in a (useful) Parsing exception and 400 rest status describing the error, rather than a 500 error.

Related Issues

Fixes #18131 (together with #18161)

Check List

• Functionality includes testing.
• [ ] API changes companion pull request created, if applicable.
• [ ] Public documentation issue/PR created, if applicable.

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[github-actions]

❌ Gradle check result for 9519cf9: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❌ Gradle check result for f7f7326: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

✅ Gradle check result for adde2dd: SUCCESS

[github-actions]

❌ Gradle check result for 58d0fda: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❌ Gradle check result for 58d0fda: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

✅ Gradle check result for 58d0fda: SUCCESS

[opensearch-trigger-bot]

The backport to 2.19 failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/OpenSearch/backport-2.19 2.19
# Navigate to the new working tree
pushd ../.worktrees/OpenSearch/backport-2.19
# Create a new branch
git switch --create backport/backport-18194-to-2.19
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 379e4643fe13740f6ef7075de98de53fe05902c8
# Push it to GitHub
git push --set-upstream origin backport/backport-18194-to-2.19
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/OpenSearch/backport-2.19

Then, create a pull request where the base branch is 2.19 and the compare/head branch is backport/backport-18194-to-2.19.

[opensearch-trigger-bot]

The backport to 2.19 failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/OpenSearch/backport-2.19 2.19
# Navigate to the new working tree
pushd ../.worktrees/OpenSearch/backport-2.19
# Create a new branch
git switch --create backport/backport-18194-to-2.19
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 379e4643fe13740f6ef7075de98de53fe05902c8
# Push it to GitHub
git push --set-upstream origin backport/backport-18194-to-2.19
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/OpenSearch/backport-2.19

Then, create a pull request where the base branch is 2.19 and the compare/head branch is backport/backport-18194-to-2.19.